Fujifilm X100S Digital Split Image focus - how it works

The Fujifilm X100S is the latest in a recent rush of cameras to include phase-detection elements on its imaging sensor, giving an AF system that is a hybrid of contrast and phase-detection methods. However, Fujifilm also uses this system to provide a unique and incredibly clever manual focus aid - which could finally allow digital cameras to offer the speed and convenience enjoyed by manual-focus SLR and rangefinder users. Fujifilm UK has posted a video showing 'Digital Split Image' focusing and Japanese camera site DCWatch has published details that allow us to show how it works.

What Digital Split Image focusing offers

The X100S's Digital Split Image system splits the central section of the camera's live view into four black-and-white stripes of the scene. These stripes line-up when the camera is in focus - in a similar way to a split prism viewfinder on a manual-focus film SLR. This gives a method of achieving manual focus while retaining a view of the entire scene, making it possible to assess focus and framing at the same time. This speed and convenience is what has helped focus peaking (a feature the X100S also offers) become a highly desirable feature in mirrorless cameras.

How it works

The best way of understanding how the system works is to understand how on-sensor phase detection works. Because the Digital Split Image focusing system is essentially presenting the camera's phase detection information visually.

Fujifilm was the first company to offer on-sensor phase detection in one of its cameras, when it launched its F300 EXR and Z800 EXR compact cameras. When they were launched, we explained how the system works. The key thing is that the light entering the left-hand-side of the lens will only give the same image as the light from the right-hand-side when the lens is in focus. So if you can 'look at' the left- and right-hand sides separately, you can compare the two images and calculate how much you need to move the lens to get them to match up.

Roll-over the buttons below to show what happens as you focus the lens:

Back Focus

In Focus

Front Focus

In this simplified schematic, you can see what happens to the image cast by the light passing through the left (blue dotted line) and right (red dotted lines) sides of the lens.

When in focus, the light from both sides of the lens converges to create a focused image. However, when not in focus, the images projected by two sides of the lens do not overlap (they are out of phase with one another).

Of course this is a massively simplified diagram with a single, vertical straight line as the subject (and no inversion of the image as it passes through the lens). The point is that we can derive information about focus if we can separately view light coming from opposite sides of the lens.

How does a phase detection sensor 'see'?

And we don't need the whole image to do this. Think about a strip of pixels taken from the sensor in the previous diagram. If you could make one such strip that receives light only from the left hand side of the lens and another that 'looks' only to the right-hand side of the lens, then you have enough information to find focus.

By comparing images from just these two strips it's possible to work out not only how far but also in which direction the lens needs to be moved to bring them into phase.

Back Focus

In Focus

Front Focus

As the slide published by DCWatch shows, the Digital Split Image system works by showing these left and right-looking images in the camera's live view - allowing you to visualize what the phase-detection system usually 'sees.' The first and third stripe shown in the viewfinder shows the image being gathered by the left-facing pixels while the second and fourth stripe show what the right-facing pixels are showing. Matching them up (bringing them into phase), brings the image into focus.

The focus region is black-and-white because the left- and right-facing pixels are all 'green' sensor elements, so aren't capturing any information about any of the other colors in the scene.

If the feature works in practice (and our brief hands-on experience is positive), it's a feature we expect to see in an increasing number of cameras - particularly mirrorless models, many of which still lack a quick method for assessing manual focus. Sadly, because they lack phase-detection elements, it's not a feature Fujifilm can add to its existing X-Pro1 and X-E1 models.

Comments

It is such a shame the Fuji split image system does not work the same as the old film way, I tried the Fuji at a show and found the split image implementation to be very messy as the image is out of focus until lined up (obviously as only sees through the lens). The whole point of the old split image was that you could see two pin sharp images moving into line as you focused so critical focus was easy as you lined up an edge in view, when the two "parts" are blurred its basically a bit of a naff split image system. I understand the only way to implement this is to use a real optical system, so why call it what it isn't?

That said, I'm wondering about the statement "many ...[mirrorless models] still lack a quick method for assessing manual focus."

Which models?

I have "crude" manual focus (no split-screen, no focus-peaking) on my Panasonic GH2, and I don't find it lacking. I can focus on an eyelash quickly and accurately. What is missing from current technology? Ordinary old zoom-in manual focus, which is not color blind, might be more accurate in more situations than this manual focus.

This is an appeal to nostalgia, in my opinion. If there is an increase in functionality over current systems, I'm missing it.

I wonder why doesn't this model have a standard rangefinder focusing mechanism. This would be nicer than an electronic one, although this method of manual focus would be only available through the viewfinder.

Anyway, this electronic digital split image system would be very useful in low-light situations if it can do IR capturing.

Fuji could and should have included this new version of its X-Trans CMOS II sensor on the only just on sale X-E1 model. Obviously they will include this sensor with associated focusing improvements in the next version of the X-E1 (the X-E1S?).

What this means is that I, and others who know this, will now have to postpone buying an X-E1 - which is ANNOYING, but what is the point in buying the newly announced X-E1 when we know what it will be replaced with?

Really there is no excuse for not putting the new sensor into the current X-E1. Come on Fuji, you know you want to .... so why lose sales in the meantime? After all some people might buy something else instead, if they 'can't wait' .......

Dear Pix Man,I am assuming you are interested in a real answer and you are not just venting. Fuji could have many reasons to do as they did, one being that both cameras are developed interleaved by different teams to increase the number of new cameras developed in a given time and one sensor was available early enough to be used in this model and the other was not. I also feel sad for Fuji Engineers that even when creating the most inspiring photographic tools they are still met with people complaining about their work. For myself, I am most impressed with Fuji's latest cameras!

I agree with you that Fuji do an excellent job of making cameras and I appreciate your point. I suppose realistically I would be unlikely to notice much difference in performance between the present X-E1 and whatever follows it. It just seems a shame that with so little time between the X-E1 and this announcement, that Fuji couldn't wait to incorporate the new sensor into the X-E1.

I think Fuji certainly, but others too, are too keen to get a product to market, for example the flaky firmware in the original X100 must have lost them sales. Refining the firmware, as they did, before introducing a camera, might in the long term give them more sales.

Purely from a practical view of how people behave in real life, others too will find irritating the knowledge that we know what will replace the newly created X-E1 now. I'm sure it is a damn good camera and I essentially applaud Fuji's approach and well thought through product strategy. Perhaps their 2 teams should merge?

Love the technology, more functional than good old circular split image focusing screens. Even better, does not black out in low light! (Also liked "Highly Confidential" classification on a slide aimed at the widest possible audience.)

What led me to write the original comment was the following statement from this page: " Fujifilm was the first company to offer on-sensor phase detection in one of its cameras". Thanks Mr. Butler for clarifying the differences in the two AF technologies in question.

I am just curious to find out if the X100s is now finally able to lock focus on horizontal lines im landscape orientation?

Although the way they have implemented the split screen mode I am afraid that it still wont't be abel to do that (just like the X100, XE-1 and X-Pro 1)Maybe DPREVIEW can check on this during the in depth review?

Yes that is how I work it and also the tip that I gave in my blog. But the X-Cameras AF seem to be troublesome for "newer photographers" who grew up with DSLR's or face detection AF cameras and never used analog split center focus screens.

If Fuji would overcome this issue (maybe the phase detection pixels are already the answer?) than the X-Cameras would become even more popular :)

Andy, it can be a problem if you try to focus on eyes during events and weddings. Bride and Mom next to each other and no focus lock on the eyes (eyelids = horizontal lines) possible.

I have found my workarounds as mentioned in my blogpost earlier. But this still seems to be an issue for a lot of photographers. The amount of visitors to my plogpost concerning these Fuji X-Camera AF tipps are an indication for that.

It seems as if some other cameras cope with it better... (i.e. Olympus OM-D, Sony NEX)

Almost every other mirrorless camera announced in the last six months has a hybrid focus system with on-sensor pdaf elements. Nikon showed what outstanding focus performance could be achieved this way with the first Nikon 1 cameras 18 months ago. So no, x-trans is not needed to make this work. Any camera with sufficient pdaf elements on the sensor could do it, and that includes the many hybrid sensors with Bayer arrays. X-Trans arrays increase the number of green sensels only slightly, from 1/2 the total to 5/9.

You can do PDAF with any of them, but you can't display a shifted image in the way that this one does; one in nine pixels in the entire central area is masked for PDAF instead of being focused in lines.

I played the little video over and over again and failed to see why it might be quicker or more effective when used to manual focus than contrast detect focus peaking. Maybe auto-focus phase detect might be lightning fast but any manual focus relies on humans twiddling a lens to focus it. Furthermore I am supposing it only works on the centre of the screen - you have to point the camera directly at he object you wish to focus. Contrast detect focus peaking shows points in focus over the entire screen area and also can show the dof actually in focus if the photographer pays attention.

I don't think I said it's quicker (though CarVac is right that, in theory, you could learn how far you'd have to move the lens to correct any given phase difference, like the AF version does).

The point is that (like focus peaking) it allows you to asses focus and framing at the same time, which is faster than magnified live view. Focus peaking also has precision issues (at least with current screen resolutions), with fast lenses. It remains to be seen if this does (and it may).

Have you ever used primarily a manual-focus only SLR with a split-image focusing screen? Once you get to know the relationship between focusing rotation and offset, focusing instantly becomes second nature.

Sony RX1 has a focus peaking contrary to what people say. It just works a bit different from RX100, NEXs. In RX1, the focusing peaking becomes visible only in the magnification mode. That is, as soon as one rotates the focus ring, it automatically zooms and displays focus peaking.

I don't think so, focus peaking main advantage is to provide a very viewable and precise focus information to aid MF ( or DMF in case of AF ), both on the LCD and EVF. You can always quickly compose after fine focusing ( which is a most usual way even with AF ). IMHO good composition is to be a photographer's gift, with very little dependency on the camera's LCD, EVF, OVF, etc, system.

Good feature, I see here the focusing square in the center only. Is this square flexible, so that you can select the area? On the other side I prefer Sony's peaking feature, you need no point to select, you just see on the color aura where the sharpness is. With super wide angle lenses it is tricky, but you get into iot quiet fast. This Fuji system will need a short time to get into it too, but after a short while it will make you focus manually quiet fast. Despite this the X100 remains a (too) expensive camera. For that price I get a NEX-6 with a second lens.

Autofocus in mass produced cameras was popularized as a sales aid, not a focusing aid and there was nothing about AF that applied specifically to wide angle lenses--in fact, it made a very poor demo with a wide angle lens. People who were doing critical work had managed to focus accurately even before autofocus.

This digital split-image is a very interesting feature for Fuji, looks very similar to analog manual SLRs. Anyway, for manual focussing, IMHO Sony's "Peaking" feature that most of their new cameras ( NEX, SLTs, RX100, RX1 ) already have, seems to be unbeatable.

In response to doreme's post - I should clarify that it's normal view focus peaking that's been removed from the RX1. It's still available in magnified live view mode, which allows the required precision but forgoes the ability to focus and compose at the same time.

Get yourself eyeglasses that focus at the correct distance for your cameras LCD screens (measured for comfortable arm position). Bifocal or Trifocal should do the job and still allow you, normal reading, camera LCD, and long distance vision.

Dan, it's not just age. Why put digital split image on the LCD and not the OVF? What is the purpose of the OVF anyway other than framing if you can't use it to manually focus the lens? If you want to manually focus the camera, you have to use it like an SLR or a point and shoot digital, obviating the reason for the OVF.

I agree with roger, this function needs to be implemented in the OVF. When it hits the X-Pro, especially, as there is lots of wonderful glass out there that would be great to use via the OVF, but there is no way to other than by hyper focal guessing.

The X-Pro's hybrid OVF is useless with any non-FUji glass. It makes no sense how they seem to be wasting the OVF. Shame.

My view on EVF is similar to rogerhyam. It would not be so bothering to wear reading glasses everywhere with me, but I still much, much prefer to shoot with camera supported against my forehead - it is not only more comfortable (for me), but better for un-blurred IQ. And optical finder - as roger says - is much better feeling of reality, colours, real detail resolution, fluent - not jerky and lagging reality, space deep reality. Artificial LED light of EVF and slow refresh rate of today's EVFs hurts my eye with conjuctivity, after longer time of shooting. The last, but not least favourable effect of RF style OVF is that it displays more than 100% of image with composition frame - this is very helpful and pleasant for composing. For me - camera without built-in viewfinder is no-camera, camera with built-in EVF is half-camera.We are different, I know there are opposite views, but that's OK, Fuji has now hit the target for me.

And one more small but pleasant detail: Fuji has placed the viewfinder on the right place - the left upper corner - one doesn't have to be Cyrano d' Bergerac to appreciate this. All these details shows that Fuji designers are photographers, not only electronic techies.

Excuse me, Lucas_ but the Fuji X100 (not "s") is one year older than Sony NEX-7, so Fuji was first on that. In any case, it was the position of the OVF in Leicas and compact cameras of many brand in the age of film, so Fuji (and later Sony) only copy, or borrow, that desing.

perhaps it wasn't possible due to pixel layout, but rather then horizontal, a 45 deg diagonal split image would have been a lot more practical as it would work equally well on vertical and horizontal lines

A question about the phase detection sensor rows / pixels: Do the same pixels get used to form the image information or are these effectively holes in the sensor as far as image capture is concerned that similarly to mapped bad pixels need to be interpolated from surrounding pixels? Or are they around the edges of the sensor?

I would assume that they can use these sensels with some clever interpolation: they have to rely on the three surrounding green pixels and perhaps also the surrounding pixels that look in the opposite direction to avoid repeated patterns in defocused areas. Or that's what I would do if I were writing a demosaicing algorithm for this sensor.

This makes me wonder what layouts other camera manufacturers use and how they deal with the missing data; I remember reading somewhere that the Nikon 1 cameras use lines of sensels dedicated to phase detection, and that you can't distinguish the phase detection sensels on Canon's sensors.

'we use several tens of thousands of pixels in the center area of a CCD, which is a very small number of pixels compared to the 12 megapixels used for imaging.' And, he says, they don't simply go to waste when taking pictures: 'sometimes they are used to compose image data and sometimes not, depending on the situation.'

The thing is, given the fact that they can do this split-view means that in the region covered by the split-image focus (1/4 of the area of the sensor), one in nine sensels sees somewhat less than half of the lens. This is continuous across the entire zone, unlike the Nikon 1. That's significantly more than "tens of thousands", it's (1/9)*(1/4)*16MP = 0.44 million sensels.

Probably not too big of a deal. There are still plenty of adjacent green sensels from which to interpolate. In an X-Trans sensor, 20/36 sensels are green in a 6x6 block, so losing four to pdaf might not even be noticeable. For areas that are in focus, the pdaf sensels could be used, though they will be dimmer and have to be boosted, adding a bit of noise. I suspect that's what Fuji means when they say they sometimes use the sensel info - when those sensels are in focus.

Is this feature available in the EVF as well, or just the back screen? For that matter, what about focus peaking? If one/both isn't useable via EVF, I'd probably just get the X20 and wait for these new MF aids to mature and also become available in their ILC offerings...

Wow, can't wait for your review of this and the X20 then. Had an X-Pro1 and sold it after finding out all i really needed and I really wanted was an X100...was going to buy it in February but guess I'll be per-ordering the X100s instead. May have to sell my X10 now too...depends on how good a shrunken X-Trans sensor performs.